The present invention relates to a method and apparatus for controlling the position of a floating rig, particularly, to a method and apparatus for controlling the position of a floating rig joining a rig and a well head positioned on the sea bottom by an excavation riser.
Where there a floating rig operating in deep water having a depth exceeding 1,000 meters, it is necessary to hold upright an excavation riser joining the rig and a well head positioned in the sea bottom. Used nowadays for this holding operation is a dynamic positioning system (DPS) for holding the excavation riser at a predetermined position by using thrusters or a combination of thrusters and a propulsion system. In this case, the information on the position of the rig is obtained from signals supplied from acoustic equipment or artificial satellites for performing a desired control. What should be noted in this case is that, if the information on the position of the rig ceases to be received, it is impossible to perform control to maintain the position. What should also be noted is that, if the marine weather conditions are aggravated, the acoustic signal looses its reliability and Differential Global Positionig System (DGPS) signal supplied from the artificial satellite tends to be interrupted. Such being the situation, employed in general is a system in which the position information is obtained by at least three sensor systems and by at least two kinds of sensors differing from each other in the principle of measurement.
Where the water depth is not larger than 300 meters, the anchor at the sea bottom and the floating rig are joined to each other by a wire (taut wire). In the position control of the rig in such a low water depth, the angle of inclination of the wire is changed by the movement of the rig. Therefore, the rig position is maintained by operating the system according to a signal recieved representing a change in the angle of inclination. The tension of the wire is adjusted if necessary.
However, in a sea having a water depth exceeding 1,000 meters, the wire joining the floating rig and the sea bottom is drifted by a current, making it impossible to control effectively the rig position. Also, a system, in which the rig position is controlled by receiving a signal representing the angle of inclination at the lower end of the riser, is employed in place of the wire. In this system, however, it is said to be impossible to perform an effective positioning control in a deep sea having a water depth exceeding 1,000 meters.
Further, the actual operation carried out is such that the angles of inclination at the upper and lower ends of the riser are perceived visually by the operator and the control target position of DPS is determined manually by the operator.
Still further, a shaft of an excavation drill is housed inside the excavation riser, making it necessary to diminish the angle of inclination of the riser to prevent the riser and the excavation drill shaft from interfering with each other. In the conventional apparatus, however, the position is not controlled depending on the angle of inclination of the riser, making it impossible to perform simply the control to maintain the position, and the control is dependent on the degree of skill of the operator.